From a pure theoretical point of view, any electrolyte that solid will have extreme challenges to design / manufacture with an acceptable energy/amperage density.
At the end of the day the basic principle of the battery is to move ions between the anode and cathode, means liquid is the best, and the warmer liquid is better than cooler.
Reason of the usage of molten salt batteries in missiles and other short term/high power applications.
Not impossible, of course , but this kind of stuff will require lot of time to mature, and become economical. And safe .
I think you better do a check again. Its already around the corner and going to be offered for EVs.